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GENETICS: E. M. EAST 
with both B and G; (2) plants fertile with B but not with G; (3) plants 
fertile with G but not with B ; (4) plants fertile with neither B nor G. 
These facts were interpreted by assuming the existence of two inde- 
pendently inherited factors that inhibit the growth of pollen-tubes. 
Representing these factors by the letters B and G, the original plants 
must be supposed to have had the formulae Bb and Gg respectively, 
since it is clear that type BB and GG could never be formed. When Bb 
is crossed with Gg the four types BG, Bg, bG and bg should result, of 
which the first three should be self-sterile. Plants BG should be fertile 
with plants bg, plants Bg should be fertile with bG and bg, plants bG 
should be fertile with Bg and bg, while plants bg should be self-fertile 
as well as cross-fertile with the other three classes. Attractive as this 
theory is, it is not clearly in accord with the facts. Plants of the type bg — 
inherently self-fertile — were not found, and the other classes showed many 
discrepancies. 
Morgan^ has offered another hypothesis that fits the data from both 
plants and animals. If I have not misunderstood the meaning of his 
rather general statement of the proposition, my own theory is only 
an extension of it, laid down perhaps a little more specifically. He 
says: 
The failure to self -fertilize, which is the main problem, would seem to be 
due to the similarity in the hereditary factors carried by the eggs and sperm; 
but in the sperm, at least, reduction division has taken place prior to fertili- 
zation, and therefore unless each animal was homozygous (which from the nature 
of the case cannot be assumed possible) the failure to fertilize cannot be due 
to homozygosity. But both sperm and eggs have developed under the influence 
of the total or duplex number of hereditary factors: hence they are alike; 
i.e., their protoplasmic substance has been under the same influence. In 
this sense, the case is hke that of stock that has long been inbred, and has 
come to have nearly the same hereditary complex. If this similarity decreases 
the chances of combination between sperm and eggs we can interpret the results. 
My own work has been done with the descendants of a cross between 
Nicotiana forgetiana (Hort) Sand., a small red-fiowered species, and 
Nicotiana alata Lk. and Otto. var. grandiflora Comes, the large white- 
flowered sort conamonly known as Nicotiana affinis. Both parents were 
undoubtedly self-sterile as over 500 plants of the Fi, F2, Fs, and F4 
generations have been found to be self-sterile by careful tests. 
Several experiments were made in which selfing, crossing inter se, 
and back crossing were done on a large scale, using plants of the F2,F3 
and F4 generations which had segregated markedly in size and were of 
